Aldosterone is important in sodium and potassium hemostasis in normal and hypertensive subjects. We have identified three areas where significant unanswered questions can be at least partially resolved by experimental methods available to us: (1) There is persistent but indirect evidence for the presence of an unidentified stimulus for aldosterone production during sodium restriction and isovolemic hyponatremia. We will use isolated frog adrenal cells, maintained as primary isolates in tissue culture as a sensitive bioassay for aldosterone stimuli. We will test sera from control and test animals and humans under standardized conditions for direct evidence for this stimulus. If we find such a response, we will proceed with steps to identify the source of the stimulus and to partially identify its chemical structure; (2) We have demonstrated that aldosterone causes an increased uptake of C14 riboflavin (RF) into Flavin Adenine Dinucleotide (FAD). The response of this to antimetabolites is similar to that shown by the kaliuretic action of aldosterone to the same agents and different from that of the anti-natriuretes. This suggests that C14 RF uptake may be related to kaliuresis. If so, this would represent an important breakthrough in understanding the relationship of intracellular events following aldosterone treatment and its consequences; (3) We propose to document this relationship by the use of specific riboflavin antagonists and to use it in conjunction with measurements of Total and Free plasma mineralocorticoid activity, measured by radio-receptor binding, to examine the role of aldosterone production and action in hypertensive rat models of hypertension and in tissue culture isolates in vitro. These techniques will permit us to examine the role in human essential hypertension of altered receptor affinity to mineralocorticoids. We will also examine patients with normal circulating concentrations of the steroids but showing evidence for volume expansion, low plasma renin activity and normokalemia.